1. Field of the Invention
The present invention relates to a sewing machine utilizing a stepping motor to convert the quantity of work piece feeding, and more particularly to an improvement of home point detection for the motor.
2. Description of the Prior art
In such prior art sewing machine, a work piece feeding cam 2 having a cam groove 1 formed along the entire periphery thereof is threadably secured to a feed stepping motor 3, as shown in FIG. 1, the cam groove 1 providing a feed pitch as a diameter from the center thereof. The contactor 6 of contactor arm 5 is slidably engaged with the cam groove 1, and the stepping motor 3 is actuated to rotate the work piece feeding cam 2 in one direction so that the diametral displacement of cam groove 1 is transferred to a feed regulator 7 as data representing a change in the quantity of work piece feeding. The feed mechanism 8 of known configuration is used to set the feed pitch of feed dog 9.
Advantages of such feed mechanism are the availability of inexpensive stepping motor and the possibility of setting a precise quantity of work piece feeding.
For such sewing machine incorporating the stepping motor, however, a so-called home point correcting procedure is required to apply in order to actuate the stepping motor return back to its home point during sewing operation, and thereby checking for proper rotating position for avoidance of step out of the stepping motor. The home point of the stepping motor means zero point of the motor. This correcting procedure has been made hereinbefore simultaneously when the sewing operation is stopped, the power switch is turned ON or during conversion of quantity of work piece feeding in starting the sewing machine operation, in order to check for proper position for avoidance of step out of motor operation.
For this reason, the home point correcting procedure of the stepping motor must be completed in a brief period. To this end, a traditional device operable to rotate the work piece feeding cam only in one direction comprises a plurality of detecting positions of home point formed over the work piece feeding cam 2 and a plural number of home point detectors including light emitting and receiving elements, the number being selected to match the number of detecting positions of home point. Thus, a shield plate 13 formed over the work piece feeding cam 2 can shield the home point detectors 12 which correspond to respective detecting positions of home point. This permits the stepping motor to reduce its rotating angle in correcting its home point, whereby ensuring a speedy execution of home point correcting procedure.
However, this arrangement presents a drawback in that the machine structure itself and its control sequence tend to be complicated for the need of providing plural number of home point detectors, whereby causing the machine to an added manufacturing cost. Though an alternative arrangement has been suggested wherein only a single home point detector is provided, this approach is impractical for an actual application since the response speed is quite slow in responding to a home point correcting signal for the stepping motor.
The present invention has been made to improve several problems in a prior art, and its object is to provide a sewing machine which can carry out the home point correction of stepping motor in an accelerated speed through single home point detector, without complicating a control sequence and increasing a manufacturing cost.